We will determine the carbon dioxide level that is required to sustain rhythmic breathing during non rapid eye movement (NREM) sleep in normal subjects and in patients with congestive heart failure. We will also determine the effect of hypoxia, hyperoxia, and theophylline on the hypocapnic apneic threshold during sleep. We hypothesize that the proximity of eupneic PCO2 to the hypocapnic apneic threshold is a critical determinant of rhythmic breathing during sleep.